1. Field of the Invention
The present invention relates to digital rotary servovalves and more particularly to a rotary to linear converter comprising a pressure preloaded spool drive thread for the servovalve spool.
2. Description of the Prior Art
Rotary to linear converters for rotary servovalves have been well known in the art. Usually the rotary servovalve has a rotary input that is connected to a stepper motor which rotates in controllable incremental steps, and moves a spool linearly to cover and uncover ports to control hydraulic flow under pressure. The spool must move under control in both directions with minimum backlash, for precise control of a closed loop system having an actuator that feeds back signals to the stepper motor. The stepper motor then is controlled by some independent program source.
The use of helical threads for converting rotary to linear movement in rotary servovalves is not new in and of itself. For example, the devices shown in U.S. Pat. No. 3,530,764, issued Sept. 29, 1970 to Kabushiki et al.; U.S. Pat. No. 3,760,687, issued Sept. 25, 1973 to Inaba et al.; U.S. Pat. No. 3,752,038, issued Aug. 14, 1973 to Inaba et al. and U.S. Pat. No. 4,031,811, issued June 28, 1977 to Inaba et al., show thread-type rotary to linear translators, but not incorporating the features of the present invention.
U.S. Pat. No. 3,899,956, issued Aug. 19, 1975 to Olsen et al. shows a system using a rotary servovalve of the general type contemplated herein under control of a stepper motor. However, in the Olsen et al. patent a spring loaded nut is used to provide take-up, which can give some problems in stability and accurate operation.
U.S. Pat. No. 3,695,295, issued Oct. 3, 1972 to Olsen et al. also relates to servovalves operated as a digital servovalve with rotary to linear translators operated by a stepper motor.
U.S. Pat. No. 4,235,156, issued Nov. 25, 1980 to Olsen et al. includes a servovalve operated by a digital controller.